Aquatics Unlimited: Articles: How Big is Your Tank - Really?
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How Big is Your Tank - Really?

for the week of 5/21/98

Many hobbyists are unaware that aquarium manufacturers name their products by nominal size, rather than by actual capacity, much like lumber yards call a piece of wood that is 1.5" thick and 3.5" wide a "2-by-4". The basic formula for determining the actual capacity of an aquarium is:

Length x Width x Height / 231

All measurements should be in inches, and the result is U.S. gallons. To accurately measure real capacity, inside measurements should of course be taken.

A common "55 Gallon" tank measures 47.5 inches long, 12 inches wide and 19" high (all inside dimensions) - yielding a maximum capacity of less than 47 gallons! Adding a couple of inches of gravel at the bottom, not filling the tank quite up to the top rim and tossing in a couple of pounds of decorative rock could easily drop the water volume in that "55 Gallon" aquarium to less than 40 gallons!

So how does the manufacturer come up with their figures? Generally, they use that same formula, but start with outside dimensions - including the frame. As such, that same "55 Gallon" tank measures 48.25 x 12.75 x 21 - for a total imaginary capacity of just under 56 gallons.

All major aquarium manufacturers appear to have used these nominal sizes for decades, and I'm sure each would claim that their competitors force them into using these exagerrated figures. Who would buy a "47 gallon" tank from brand A if they could get a "55 gallon" from brand B for the same price - even if they had exactly the same dimensions? And for the most part, it really doesn't matter, as long as a 55 is bigger than a 45 is bigger than a 38 and so on.

The one instance where it does matter is when medication needs to be added. Treating 40 gallons of water as if it were 55 gallons - then adding a little extra for good measure - could easily result in an overdose that could injure or kill the fish.

Submitted by: Jim Kostich


"Tip of the week" appeared regularly in 1999 and 2000.

Copyright © 1998 Aquatics Unlimited
All rights reserved.

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